Exhaust gas cleaning system for handling radioactive fission and activation gases

ABSTRACT

An exhaust gas cleaning system utilizing the principle of delaying radioactive gases to permit their radioactive decay to a level acceptable for release to the atmosphere, comprising an adsorbent for adsorbing radioactive gas and a container for containing the adsorbent and for constraining gas to flow through the adsorbent, the adsorbent and the container forming simultaneously an adsorptive delay section and a mechanical delay section, by means of a predetermined ratio of volume of voids in the adsorbent to total volume of the container containing the adsorbent, for delaying radioactive gas to permit its radioactive decay to a level acceptable for release to the atmosphere. A method of using an adsorbent for cleaning a radioactive gas containing an isotope which is adsorbed by the adsorbent and containing an isotope whose adsorption by the adsorbent is low as compared to the isotope which is adsorbed and which is shortlived as compared to the isotope which is adsorbed, comprising constraining the gas to flow through the adsorbent with the retention time for the isotope which is adsorbed being at least the minimum for permitting radioactive decay to a level acceptable for release to the atmosphere and with the retention time for the isotope of relatively low adsorption and relatively short life being at least the minimum for permitting radioactive decay to a level acceptable for release to the atmosphere.

United States Patent Queiser et al.

[111 3,871,842 5] Mar. 18,1975

1 EXHAUST GAS CLEANING SYSTEM FOR HANDLING RADIOACTIVE FISSION ANDACTIVATION GASES {75] Inventors: Horst Queiser, Hochstadt; HorstSchwarz, Wiesbaden, both of Germany [73] Assignee:Licentia-Patent-Verwaltungs G.m.b.H., Frankfurt, Germany [22] Filed:July 5, 1972 [21] Appl. No.: 269,272

[30] Foreign Application Priority Data July 5, 1971 Germany 2133250 July5, 1971 Germany 7125651 [51] Int. Cl. 801d 53/04 [58] Field of Search55/66, 74, 387, 179, 58;

[56] References Cited UNITED STATES PATENTS 3.444.725 5/1969 Chase176/37 3,501,923 3/1970 Lehmer... 55/66 3718539 2/1973 West 176/373,720,043 3/1973 Kovach 55/74 [57] ABSTRACT An exhaust gas cleaningsystem utilizing the principle of delaying radioactive gases to permittheir radioactive decay to a level acceptable for release to theatmosphere, comprising an adsorbent for adsorbing radioactive gas and acontainer for containing the adsorbent and for constraining gas to flowthrough the adsorbent, the adsorbent and the container formingsimultaneously an adsorptive delay section and a mechanical delaysection, by means of a predetermined ratio of volume of voids in theadsorbent to total volume of the container containing the adsorbent, fordelaying radioactive gas to permit its radioactive decay to a levelacceptable for release to the atmosphere.

A method of using an adsorbent for cleaning a radioactive gas containingan isotope which is adsorbed by the adsorbent and containing an isotopewhose adsorption by the adsorbent is low as compared to the isotopewhich is adsorbed and which is short-lived as compared to the isotopewhich is adsorbed, comprising constraining the gas to flow through theadsorbent with the retention time for the isotope which is adsorbedbeing at least the minimum for permitting radioactive decay to a levelacceptable for release to the atmosphere and with the retention time forthe isotope of relatively low adsorption and relatively short life beingat least the minimum for permitting radioactive decay to a levelacceptable for release to the atmosphere.

9 Claims, 4 Drawing Figures EXHAUST GAS CLEANING SYSTEM FOR HANDLINGRADIOACTIVE FISSION AND ACTIVATION GASES BACKGROUND OF THE INVENTION Thepresent invention relates to an apparatus for cleaning radioactive gasesto permit safe disposal into the atmosphere.

The radioactive fission and activation gases set free in nuclearreactors must be treated before their release to the atmosphere. Thepurpose of this treatment is to prevent there occurring an unacceptablyhigh contamination of the environmental air around the nuclear reactors.with the unacceptable radiation load associated with such contamination.

In the treating of such exhaust gases, it is primarily a matter ofdelaying them to permit their radioactive decay to satisfactory levels.The article, Reactorabgas und Gebaudeabluftbehandlung im Kernkraftwerkmit Siedewasserreactor (in translation, Reactor Exhaust Gas and BuildingExhaust Air Treatment in the Nuclear Power Plant with Boiling WaterReactor), published in the August 1970, (Volume 16, No. 8) issue of Atomund Strom (in translation, Atom and current), pages l to l 18, shows atreatment system wherein the gas is first mechanically delayed in a longpipeline circuit, so that short-lived active substances, primarilynitrogen and oxygen isotopes, can decay. That this mechanical delayingis needed has been clear for the known treatment plants, becausenitrogen and oxygen can only be delayed adsorptively to a limitedextent.

The article gives an introduction to the basics of exhaust gasescontaining radioactive components. In the case of a boiling waterreactor, the gases which arise by activation are primarily isotopes ofoxygen, nitrogen, and argon.

Following the mechanical delay section of the treatment system of thearticle comes a mechanical filter, whose purpose is to collect the soliddaughter products arising during the decay of the active gases in themechanical delay section. Then following the filter comes a purelyadsorptive delay section, where the longerlived radioactive substancesare caught on the surface of the adsorbent by free surface forces andconsequently delayed. Of concern here are primarily only the noble gasesxenon and krypton. The solid fission products resulting in theadsorptive delay section can be active; they are separated at the end ofthe adsorptive delay section by a mechanical filter. Since theadsorptive delaying action increases with decreasing moisture an themixture to be delayed, the described system provides a gas drying stepbefore the adsorptive delay section.

The described method has the disadvantage of necssitating a largeexpense for apparatus, in order to achieve exhaust gas cleaning. This isespecially true in the case of the mechanical delay section, which isformed by a long pipeline circuit. Since condensate inherently forms inthe pipeline circuit, a complex dewatering system is necessary. In thecase of operation at pressures below atmospheric, the problems ofcondensate removal become greater.

The mechanical delay section is also caused to be large because of theparabolic flow distribution in the pipelines. The flow velocity is thushigher in the center of a pipe than at the wall. This means that theretention time for material to be delayed can take on different values.However, the designing of the delay section has to be carried out on thebasis of the flow velocity at the center of the pipes.

The large volumes associated with this prior system requires expensiveradiation shielding, both for the pipeline circuit and for the spacewhere the removed condensate is collected.

SUMMARY OF THE INVENTION An object of the present invention, therefore,is to provide an exhaust gas cleaning system for decreasing theapparatus expense as compared to the abovedescribed system; at the sametime, cleaning ability is not to be negatively influenced.

A further object is to avoid problems of condensate removal for themechanical delay section, to hold the volume of the mechanical delaysection as small as possible, and, as a result of such small volume, tohold radiation shielding expenses small.

These as well as other objects which will become apparent in thediscussion that follows are achieved, according to the presentinvention, by:

1. an exhaust gas cleaning system utilizing the principle of delayingradioactive gases to permit their radioactive decay to a levelacceptable for release to the atmosphere, comprising adsorbent means foradsorbing radioactive gas and container means for containing theadsorbent means and for constraining gas to flow through the adsorbentmeans, the adsorbent means and the container means formingsimultaneously and adsorptive delay section and a mechanical delaysection, by means of a predetermined ratio of volume of voids in theadsorbent means to total volume of the container means containingadsorbent means, for delaying radioactive gas to permit its radioactivedecay to a level acceptable for release to the atmosphere; and

2. a method of using an adsorbent for cleaning a radioactive gascontaining an isotope which is adsorbed by the adsorbent and containingan isotope whose adsorption by the adsorbent is low as compared to theisotope which is adsorbed and which is short-lived as compared to theisotope which is adsorbed, comprising constraining the gas to flowthrough said adsorbent with the retention time for the isotope which isadsorbed being at least the minimum for permitting radioactive decay toa level acceptable for release to the atmosphere and with the retentiontime for the isotope of relatively low adsorption and relatively shortlife being at least the minimum for permitting radioactive decay to alevel acceptable for release to the atmosphere.

GENERAL ASPECTS OF THE INVENTION The objects of the invention areachieved in an exhaust gas cleaning system with a mechanical and anadsorptive delay section for treating radioactive fission and activationgases, wherein the adsorptive delay section simultaneously forms themechanical delay section for the gas mixture to be treated, this beingaccomplished by appropriately choosing the ratio of the volume of voidsbetween the particles of the adsorbent making up the adsorptive delaysection to the total volume into which adsorbent is filled.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic flow diagram ofan exhaust gas treatment plant according to the invention.

FIG. 2 is a partially schematic elevational view of a main adsorber ofthe present invention.

FIG. 3 is a partially schematic elevational view of a preliminaryadsorber of the invention.

FIG. 4 is a partially schematic plan view of a plant according to theinvention in a vacuum chamber.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As brought out in theBACKGROUND OF THE IN- VENTION, radioactive gas to be cleaned accordingto the invention can contain nitrogen and oxygen as isotopes which arerelatively unadsorbed and of low halflife, while containing xenon andkrypton and isotopes of relatively greater half-life which are adsorbed.

It is preferred to provide the system of the present invention with apreliminary adsorber. with the gas mixture to be treated being firstdelayed adsorptively there and then being filtered for removal of soliddaughter products resulting from active gas decay. Then, in a mainadsorber, the remaining, longer-lived isotopes are delayed adsorptivelyand mechanically, with the resulting solid daughter products being againfiltered off.

An example of the invention will now be described with reference to thedrawings, firstly with reference to FIG. 1. The radioactive fission andactivation gas to be treated is sucked out of a turbine condenser (notillustrated) by evacuation pump 1 and, in mixture with the drivingsteam, fed to the exhaust gas treating plant. After a slightsuperheating of the steam/gas mixture in a heater 2, the mixture moveson to recombiner 3, where free hydrogen and oxygen in the mixture areburned to water on the surface of a catalyst, for example of platinum.The heat given off by the exothermic reaction causes a large temperatureincrease. Temperatures of 400C in the mixture are comprehended. The thusstrongly superheated steam/gas mixture goes next into a condenser 4 andthen a cooler 5. The driving steam and the steam from the radiolysis gasbeing burned to water are largely condensed and withdrawn, so that onlyan inert gas mixture, composed primarily of air with some remainingsteam, comes from cooler 5.

The flow diagram shows a second, parallel line of devices 1 to 5. Suchtechnique is used, for example, in the system described in the articlementioned above.

Following the condenser 4 and cooler 5 is a gas cooling and drying plant6. The gas leaving the cooler and dryer 6, usually will be at atemperature in the range of O-30C, with a humidity corresponding to adew point in that range. Then comes preliminary adsorber 7, and a secondpreliminary adsorber 7', which is used as needed. Following directly onpreliminary adsorber 7 is the main adsorber 8. The main adsorber caninclude one or more adsorption columns. There follows vacuum pumps 9 and9, by which the treated gas is led to an exhaust air and exhaust gaschimney 10.

The essential components of the exhaust gas cleaning system are thepreliminary adsorber 7, and perhaps 7', and the main adsorber 8. Thepreliminary adsorber, which usually represents the smaller part of theadsorp tion plant, serves essentially for the decay of the shortlivedfission products. present as the vastly greater portion of the gaseousnuclide mixture. In conformance with the high proportion at which theshortlived fission products are present, there is a correspondinglylarge heat and daughter product production. The forming solid daughterproducts are simultaneously filtered off. The preliminary adsorber isconstructed in such a manner that the adsorbent, and the solid daughterproducts, which deposit in the adsorbent and are thus filtered offsimultaneously, can be exchanged easily and without danger. Furtherdetails will become apparent with the description of FIG. 3 below.

The main adsorber, which usually represents the larger part of thesystem, serves both for mechanically and adsorptively delaying theremaining, longer-lived isotopes and for the filtering of the resultingsolid daughter products. The container of the main adsorber 8 is,therefore, of larger volume than the preliminary adsorber and isprovided with simple installations for an improbable, yet conceivable,exchange of the adsorbent.

By choosing a suitable ratio of volume of voids in the adsorbent to thetotal volume of container containing adsorbent, which ratio may becontrolled for example by the particular shape or size of adsorbentparticles used, it is possible to provide in main adsorber 8 for thefurther decay of poorly adsorbable gas components in the voids betweenthe adsorbent packing bodies.

The main adsorber 8 can be provided in the form of a cylindrical vesselhaving a floor on which the adsorbent lies. Gas flow-through can beeither from below upwards, or from above downwards. An especiallyadvantageous embodiment of the main adsorber is illustrated in FIG. 2. Acentral tube 12 is arranged in standing container 11; tube 12 dividesthe container 11 into two spaces 13 and 13 for receiving adsorbent 50,for example activated carbon. These spaces are flowed through, one afterthe other. Depending on particular circumstances, more central tubes 12of other diameters can be arranged concentrically in the standingcontainer 13, so that a larger number of separate spaces is provided forreceiving adsorbent. The locations of gas entrance and exit connections,and filling and emptying connections for adsorbent, can be seen likewisein FIG. 2.

In order to catch solid particles which might be carried out of the mainadsorber, no additional filter is needed. Rather, this catching is doneby using liquidpiston type rotary blowers to forward the gas. The liquidneeded for blower operation then catches any low activity solidimpurities present in the gas. The separation of the largely decayed andcleaned gas from the liquid proceeds in a gas-water separator. Asuitable liquid-piston type rotary blower is shown in FIGS. 6-47 on page6-24 of PERRYS CHEMICAL ENGINEERS HANDBOOK, McGraw-Hill, New York, 1963.

A preferred embodiment of the preliminary adsorber 7 (or 7) is shown inFIG. 3. Included are a cylindrical upper part l4 and a conical lowerpart 15. Pipe 16 extends from above down into the interior of thechamber formed by parts 14 and 15; gas enters from this pipe intoadsorbent 51, for example activated carbon. Lid 20 closes the top of thechamber. A gas distributor 17 on the end of pipe 16 provides for theemission of the gas into the adsorbent while at the same time preventingan encroachment of the adsorbent from the area between the outside ofpipe 16 and the inner walls of parts 14 and 15, into pipe 16. Forreasons of radioactivity and easy filling and emptying, the preliminaryadsorber 7 is arranged between an upper shielding floor 18 and a lowershielding floor 19. A filling pipe 21, for the filling of packing bodiescarrying adsorbent, is accessible from above the upper shielding floor18 and opens into the adsorbent chamber through lid 20. For the emptyingof adsorbent, a discharge tube 22 extends downwardly from the lower endof conical part 15, through lower shielding floor 19, into the spacebeneath. Either a part, or all, of the adsorbent can be extracted fromthe adsorbent chamber through tube 22 and into one or more transportvessels. Control of the adsorbent emptying is accomplished by a pushgate 23 in the discharge tube 22. Because the pipe 16 extends down intothe conical lower part 15, it is possible to empty first always the moststrongly loaded adsorbent through discharge tube 22. Thus, that portionof the adsorbent in the vicinity of the gas distributor 17 provides thegreatest filtering and retention of solid daughter products. Freshadsorbent can then be added for makeup purposes through filling pipe 21.

A transport car 24 with vessel 25 can serve for receiving and carryingaway the emptied adsorbent. Sealing between the vessel 25 and dischargetube 22 is done using a plastic sack according to conventional weldingtechniques.

As indicated by the arrows in FIGS. 3 and 2, a radioactive gas to becleaned first flows from the cooling and drying plant 6 in FIG. 1 intopipe 16 in FIG. 3, out into the adsorbent 51, upwards in part 14, andout through pipe 52 in lid 20. Pipe 52 is connected to pipe 53 in FIG.2. Gas flowing into pipe 53 flows upwards through space 13' and thenturns down through the outer, annular space 13, finally to leave, incleaned state, through pipe 54. Adsorbent is filled into space 13'through connection 55a; into space 13 through connection 55b. Adsorbentmay be removed from space 13 through connection 56b and from space 13'through connection 56a.

The method of the present invention provides longer delay times ifpressures above atmospheric are used and also if cooling is provided.But then, the use of a below-atmospheric pressure chamber can improvethe exhaust gas cleaning system still further. Such is illustrated inFIG. 4. Thus, usually, radioactive exhaust gas cleaning systems areoperated at pressures below atmosphcric for reasons of safety.

The below-atmospheric or vacuum chamber includes a concrete housing 30for the purpose of providing shielding from radiation. The inner wallsofthis housing are lined with a heat insulation 31. The vacuum chambersurrounds the main adsorber 8, which in this embodiment includes threeadsorption chambers 32, 32, and 32". The vacuum chamber is accessiblethrough an air lock 33, which is closed during normal operation.Ventilation of the vacuum chamber is done using an air introduction line34 and an air extraction line 35.

The air sucked out of the vacuum chamber, as well as the air which isintroduced, is passed through an ultrafine filter 36 and an activatedcarbon filter 37 by means of a blower 38, whose suction side facesfilter 37, in conformance with the arrows indicating air flow in thevacuum chamber. On the pressure side of blower 38, throttle valves 39and 40 are adjusted with respect to one another to recycle a portion ofthe air while sending the remainder to the chimney 42. The air recycledto the vacuum chamber can be sent through a cooler 41, as shown, withthe gas in line 34 being in the range 2()C to +C. The throttle valve 57controls the admission of fresh air to the system.

FIG. 4 further shows that the gas mixture to be treated in the mainadsorber 8 is first passed through a compressor 43, whose pressure sidefaces chamber 32 in the drawing. Upon leaving the main absorber, the gasmoves through an expansion valve 44 and thence out of the vacuum chamberand into chimney 42. In this system the pressure within chamber 30 is inthe range 0.95-1 atm. and the pressure downstream of compressor 43 is inthe range 0.8-20 atm.

A significant advantage of the exhaust gas cleaning system of thepresent invention resides in its elimination of a separate mechanicaldelay section, so that the total system becomes cheaper and lessdemanding of space. Since the mechanical delay takes place after thecooling, condensing, and drying provided by apparatus components 4, 5,and 6 described above, condensate does not occur during the mechanicaldelay. By filling the adsorbers with adsorbent, a substantially constantgas flow velocity is obtained across the adsorber cross sections; thisenables an excellent utilization of the volume available for mechanicaldelay.

The space saving aspect of the present invention means that lesseramounts of radiation shielding are needed.

Because the preliminary adsorber is arranged between two shieldingfloors, it is possible to fill and empty adsorbent with safety.

The dividing of the main adsorber up into a plurality of concentricadsorbent-receiving chambers also saves space, as compared with apractice of providing the adsorbent in a plurality of separate columns.

The preferred activated carbon, which is used as adsorbent, has thefollowing specification:

particle size [-3 mm internal surface I000 m lg bulk density 0.5 g/mThis carbon is manufactured by Bergwerksverband, Essen-Kray,FrillendorferstraBe 351. The trademark name is Aktivkohle Typ VRG 1-3.

The release rates at the stack are based on the permissible exposurerates at the location of the power plant. The permissible rates both inthe United States of America and in Germany are in the range of 5-50mrm/a (millirem per annum). This means-depending on the meteorologicalconditions at the site-values at the stack-outlet of the power plant inthe range of ll00 Curie/h (see the article, mentioned on page 3, lines13 to 19.)

To design the exhaust gas cleaning system for handling the radioactivefission and activation gases there must be known the kind, the quantityand the activity of the radioactive gases to be treated in connectionwith the maximum radiation level in gases to be released to theatmosphere as mentioned above.

Examples of the kind, the quantity and the activity of the radioactivegases in connection with a boiling water reactor are given in theabove-mentioned article and also in the article: The off-gas system ofthe power plant Gundremmingen published in the May 1971 (Volume 13, No.5) issue of Kerntechnik, pages 205-213, especially Table l and 2.According to these publications, the usual flow rates for the mixture ofair, fission gases and gases formed from water by activation, are in therange of 10-80 m /h at 1 atm. In the system according to the inventionthe quantity of activated carbon is calculated on the basis of thefission gases, that is, the isotopes of the noble gases, xenon andkrypton. to provide sufficient delay time in the absorptive delaysection. Taking the absorptive coefficient for xenon at normalconditions at 600l000 em /g, the amount of carbon required for thispurpose will ordinarily be in the range of 10 to 100 tons.

The voids in the activated carbon fill constitute a mechanical delaysection for the activation gases, such as the isotopes of oxygen andnitrogen. In the case where the active carbon has properties similar tothose of the preferred form referred to above, the void volume of thebed usually is in the range of 30 to 50% which provides sufficient delaytime for the activation gases.

The filters 36 and 37 in FIG. 4 are conventional high efficiency filtersfor aerosols which are commonly used in nuclear and other industrialventilating systems. Examples of such devices are those manufactured bythe firm of Delbag in Berlin, Germany.

It will be understood that the above description of the presentinvention is susceptible to various modifica tions, changes andadaptations, and the same are intended to be comprehended within themeaning and range of equivalents of the appended claims.

It is claimed:

1. Apparatus for processing radioactive exhaust gases including fissiongases and activation gases, said apparatus comprising preliminaryadsorber means including a container, main adsorber means including acontainer, and adsorber material filling each of said adsorbercontainers, said main and preliminary adsorber means being connected toform a series delay path between a source of such exhaust gases and anexhaust gas chimney to constrain such exhaust gases to flow through saidabsorber material and to be delayed by said delay path in order toundergo radioactive decay, said delay path providing essentially theonly gas delay between the source and the chimney, said preliminaryadsorber means serving to permit radioactive decay of those exhaust gascomponents which constitute shortlived fission products and to filterout the resulting daughter products, said preliminary adsorber meansbeing provided with shielding means and comprising means permitting easyreplacement of said adsorber material.

2. A system as claimed in claim 1 wherein said main adsorber meansinclude at least one concentrically arranged tube means for dividing theinterior of said main adsorber means container into a plurality of spacemeans for receiving said adsorber material, and means for causingradioactive gas to flow through said space means one after the other.

3. Apparatus as defined in claim 2, wherein there are a plurality ofcentrally disposed pipes arranged concentrically.

4. Apparatus as defined in claim 1 wherein said container of saidpreliminary adsorber means is composed of a cylindrical upper part and aconical lower part, said replacement permitting means include adischarge opening in said lower portion for the easy and safe dischargeof adsorber material, and said preliminary adsorber means furthercomprise a gas inlet pipe extending into the interior of said conicallower part for introducing radioactive exhaust gas into said adsorbermaterial.

5. A system as claimed in claim 4 wherein said preliminary adsorbermeans further include an upper shielding floor means above said upperpart for shielding radiation emitted in said upper and lower parts, anda lower shielding floor means below said lower part for shieldingradiation emitted in said upper and lower parts, and said replacementpermitting means further include pipe means for delivering adsorbermaterial down through said upper floor means and into said upper andlower parts, and a discharge tube means for extracting adsorber materialfrom said discharge opening down through said lower floor means.

6. Apparatus as defined in claim 5 further comprising a vacuum chamberin which said main adsorber means is situated, and means connected forcontrolling the temperature of said chamber.

7. Apparatus as defined in claim 1 for processing exhaust gases whosefission gases include krypton and xenon and whose activation gasesinclude nitrogen isotopes and oxygen isotopes.

8. Apparatus as defined in claim 1 wherein said main adsorber meansfilled with adsorber material forms simultaneously an adsorptive and amechanical delay path by a predetermined ratio of the void volume insaid adsorber material to the volume of said container for said adsorbermaterial.

9. An exhaust gas cleaning system utilizing the principle of delayingradioactive gases to permit their radioactive decay to a levelacceptable for release to the atmosphere, comprising adsorbent means foradsorbing radioactive gas, said adsorbent means being divided into twoportions, a preliminary adsorber containing one of said portions, fordelaying radioactive gas and for filtering out solid daughter products,and a main adsorber containing the other of said portions, for furtherdelaying radioactive gas which has passed through said preliminaryadsorber means and for further filtering out of solid daughter products,said preliminary adsorber including a cylindrical upper part, a coaxialconical lower part connected to and tapering narrower downwardly fromthe cylindrical upper part, said conical part having means for adsorbentremoval, and a pipe opening into the interior of said conical lower partfor the introduction of radioactive gas into said adsorbent means, anupper shielding floor above said upper part for shielding radiationemitted in said upper and lower parts, a lower shielding floor belowsaid lower part for shielding radiation emitted in said upper and lowerparts, means for filling the portion of said adsorbent means associatedwith said preliminary adsorber extending downwardly through said upperfloor into said upper and lower parts, and a discharge means extendingdownwardly from said extraction opening means through said lower floor.

1. Apparatus for processing radioactive exhaust gases including fissiongases and activation gases, said apparatus comprising preliminaryadsorber means including a container, main adsorber means including acontainer, and adsorber material filling each of said adsorbercontainers, said main and preliminary adsorber means being connected toform a series delay path between a source of such exhaust gases and anexhaust gas chimney to constrain such exhaust gases to flow through saidabsorber material and to be delayed by said delay path in order toundergo radioactive decay, said delay path providing essentially theonly gas delay between the source and the chimney, said preliminaryadsorber means serving to permit radioactive decay of those exhaust gascomponents which constitute short-lived fission products and to filterout the resulting daughter products, said preliminary adsorber meansbeing provided with shielding means and comprising means permitting easyreplacement of said adsorber material.
 2. A system as claimed in claim 1wherein said main adsorber means include at least one concentricallyarranged tube means for dividing the interior of said main adsorbermeans container into a plurality of space means for receiving saidadsorber material, and means for causing radioactive gas to flow throughsaid space means one after the other.
 3. Apparatus as defined in claim2, wherein there are a plurality of centrally disposed pipes arrangedconcentrically.
 4. Apparatus as defined in claim 1 wherein saidcontainer of said preliminary adsorber means is composed of acylindrical upper part and a conical lower part, said replacementpermitting means include a discharge opening in said lower portion forthe easy and safe discharge of adsorber material, and said preliminaryadsorber means further comprise a gas inlet pipe extending into theinterior of said conical lower part for introducing radioactive exhaustgas into said adsorber material.
 5. A system as claimed in claim 4wherein said preliminary adsorber means further include an uppershielding floor means above said upper part for shielding radiationemitted in said upper and lower parts, and a lower shielding floor meansbelow said lower part for shielding radiation emitted in said upper andlower parts, and said replacement permitting means further include pipemeans for delivering adsorber material down through said upper floormeans and into said upper and lower parts, and a discharge tube meansfor extracting adsorber material from said discharge opening downthrough said lower floor means.
 6. Apparatus as defined in claim 5further comprising a vacuum chamber in which said main adsorber means issituated, and means connected for controlling the temperature of saidchamber.
 7. Apparatus as defined in claim 1 for processing exhaust gaseswhose fission gases include krypton and xenon and whose activation gasesinclude nitrogen isotopes and oxygen isotopes.
 8. Apparatus as definedin claim 1 wherein said main adsorber means filled with adsorbermaterial forms simultaneously an adsorptive and a mechanical delay pathby a predetermined ratio of the void volume in said adsorber material tothe volume of said container for said adsorber material.
 9. An exhaustgas cleaning system utilizing the principle of delaying radioactivegases to permit their radioactive decay to a level acceptable forrelease to the atmosphere, comprising adsorbent means for adsorbingradioactive gas, said adsorbent means being divided into two portions, apreliminary adsorber containing one of said portions, for delayingradioactive gas and for filtering out solid daughter products, and amain adsorber containing the other of said portions, for furtherdelaying radioactive gas which has passed through said preliminaryadsorber means and for further filtering out of solid daughter products,said preliminary adsorber including a cylindrical upper part, a coaxialconical lower part connected to and tapering narrower downwardly fromthe cylindrical upper part, said conical part having means for adsorbentremoval, and a pipe opening into the interior of said conical lower partfor the introduction of radioactIve gas into said adsorbent means, anupper shielding floor above said upper part for shielding radiationemitted in said upper and lower parts, a lower shielding floor belowsaid lower part for shielding radiation emitted in said upper and lowerparts, means for filling the portion of said adsorbent means associatedwith said preliminary adsorber extending downwardly through said upperfloor into said upper and lower parts, and a discharge means extendingdownwardly from said extraction opening means through said lower floor.